Silification of the Mesozoic Rocks Accompanying the Bełchatów Lignite Deposit, Central Poland

Abstract

Fieldwork and exploratory study of Poland’s Bełchatów lignite deposit reveals that the Jurassic and Cretaceous sediments with overlying Neogene clays include rocks of greater hardness than the primary composition would indicate. Mineralogical and petrographic tests show the impact of secondary mineralization involving silification in particular. Transitional and carbonate rocks observed microscopically and subjected to X-ray examination show numerous polymorphic forms of silica replacing carbonate minerals. Opal types A and CT, chalcedony, quartz and microcrystalline quartz are all present. The process of silification observed is a selective and multistage one, with selective activity entailing the displacement and replacement of carbonates from older rocks, mainly Cretaceous opoka-rocks and marls, and Jurassic limestones. The opal fills tectonic fractures and has cemented cracked grains. Cathodoluminescence analysis identifies several generations of silica. The rocks have undergone advanced diagenesis as is evidenced by the recorded metasomatic reactions between minerals. They can further be assumed to be in the locomorphic stage. Such observations are relevant to efforts to reconstruct the origin of the rock matrix, and to the study of its textural features. In addition, the tests run on rocks of the lignite series would seem to be of significant value in identifying and developing associated rocks.